In digital spread spectrum (DSS) communication, a wide band carrier signal is modulated by a narrow band message signal. The wide-band carrier is typically generated by modulating a single frequency carrier using a pseudo-random noise (P/N) code sequence. The data rate at which a message is communicated is usually much lower than the P/N code symbol or “chip” rate. The ability of DSS to suppress interference is proportional to a ratio of the chip rate to data rate. In many applications, there are thousands of code chips per data bit.
At the receiver, a carrier replica is generated by reducing the DSS signal to baseband and multiplying it with a locally generated replica of the original narrow-band carrier using a local oscillator. If the frequency and phase of the carrier replica is the same as that of the received original narrow-band carrier, then the multiplier output signal will be the product of the bipolar P/N code and intended message. The P/N code is removed by multiplying the wide-band data stream with the locally generated replica of the P/N code that is time aligned with the received P/N code. This is the de-spreading process.
Generating the carrier replica with proper carrier frequency and phase and generating the P/N code replica at the proper rate and time offset is a complex problem. In many DSS communication systems, the necessary carrier frequency, carrier phase, and P/N code offset are not known a priori at the receiver, which tries different values until a large signal is observed at the data-filter output. This is termed the search or acquisition process, and a DSS signal is said to be acquired when the proper frequency, phase, and code offset have been determined. A receiver selects and detects a particular transmitted signal by choosing the appropriate P/N code and performing the acquisition search. In some cases the acquisition search must include examination of different P/N codes from a known list when the transmitting node is not known, as is the likely scenario in FIG. 1. When many different codes, code offsets and carrier frequencies must be examined and the SNR is low, the acquisition task can be both time and energy consuming.
The above constraints are more pronounced in a secure environment such as that depicted in FIG. 1 (detailed below), where a new node termed a hailing node 34 seeks to join an existing network while maintaining security for the joining node and those nodes already on the network. In addition, an established network requires a method of discovering the existence of another separate network that may have migrated into communication range, so that a cross-link can be established between the networks in order to form a larger network. This process of nodes “discovering” each other is termed herein node discovery, and is where DSS signal acquisition occurs. Typically, node discovery is done on channels separate from the primary data communication channels. Limited data exchange on the ‘discovery channel’ is preferable for network optimization. As a result, the discovery waveform must be flexible in the messages it carries and not be constrained to one specific message type or size.
The air interface should consist of a flexible and symmetric full-duplex or half-duplex link. The transmitting node or hailing node is that node that sends a discovery burst, essentially a message inquiring as to the presence of receiving nodes. Receiving nodes are the nodes that listen for that discovery burst. The receiving nodes are therefore target nodes, which may already have formed a network. These receiving nodes may become transmitting nodes when they send an acknowledgement back to the initiating new node. In this way, a new node that flies into range of an established network will transmit burst discovery messages on that transmitting node's transmit link. When a receiving node in the established network hears the discovery message on its receive link, it will respond via its transmit link which is the hailing node's receiving link. Subsequent handshaking can then be performed via the two node's transmit and receive links to bring the initiating new node into the network. The transmitting and receiving links may occupy separate time slots in a time division duplex (TDD) system, or may be separate frequency bands in a frequency division duplex (FDD) system.
An exemplary but non-limiting environment in which node discovery may be important is illustrated in perspective view at FIG. 1, a prior art arrangement of disparate nodes operating in a traffic data network and one hailing node seeking to join the traffic network. The nodes may be airborne as in aircraft; terrestrial as in autos, trucks, and trains; or waterborne as in ships and other surface watercraft. They may be stationary or mobile, fast or slow moving, as for example, communications between nodes in a building, an aircraft, and an auto. For additional flexibility, it is assumed that a hailing node 34 may not have a clock signal synchronized with the network prior to joining. The range 22 of the traffic data network is centered on a command node 24, absent relays by other nodes within the network. Where the network links members via a satellite link, the line-of-sight range 22 is not particularly relevant. The range 22 is included to show further advantages of the invention that may be exploited when network communications are geographically limited.
The command node controls access to the network, identifying nodes and answering discovery bursts with a particularly long P/N code for data exchange to ensure networked members may communicate securely. In FIG. 1, all nodes depicted as within the traffic network range 22 communicate on the traffic network, either through the command node 24 or directly with one another once granted network entry. The traffic network typically operates by directional antennas 24a, at least at the command node 24, to maximize the network range 22. This is because directional antennas typically enable a higher antenna gain and a higher tolerable path loss as compared to omni-directional antennas. Therefore, a range (not shown) of a discovery network that operates using omni-directional antennas 24b is somewhat less, at least in the prior art. The command node 24 maintains communication with stationary nodes 26, 28. When two nodes are aircraft, they may be closing or separating from one another at very high rates, rendering Doppler effects significant. When a hailing node 34 sends a discovery burst to locate and request entry into the traffic network, its signal is typically not received at the command node 24 until the hailing node is within the traffic network range 22. Since the hailing node 34 is not yet identified as authorized, this potentially puts communications within the network at risk, or alternatively unduly delays granting the hailing node 34 access to the network. Because access to the traffic network is obtained through the discovery protocol, that protocol must exhibit security features to prevent compromise of the traffic network.
Considering the issues apparent in light of FIG. 1, a good node discovery scheme for a highly secure communications network would therefore exhibit (a) high speed and reliability; (b) long range; (c) low probability of intercept (LPI) and low probability of detection (LPD) by unauthorized parties; (d) universal discovery and recognition among the various nodes; (e) asynchronous discovery; and (f) reliability for both stationary and fast-moving nodes. As to item (a), discovery and verification of a hailing node is a key metric used to evaluate various discovery schemes. Fast network entry is desirable because it permits nodes to enter and exit networks frequently and further minimizes the time spent sending discovery bursts, reducing LPD.
As to item (b), discovery and verification should preferably be done when the hailing node is beyond the range of direct communications with the network. In secure communications without satellite links where range is limited, hailing nodes are unaware of the location of nodes within the network and vice versa, so omni-directional antennas are generally used for discovery. This is because the reduced range of communications using omni-directional antennas is more than offset by having to scan various quadrants with a directional antenna for entering nodes that may come from any direction. Once within a network, nodes use directional antennas for increased range and reduced LPI. If all discovery handshaking can occur before the hailing node is in directional antenna range, then all synchronization can be done ahead of time. This synchronization will include the synchronization of chip clocks and carrier local oscillators in the presence of potentially large Doppler and reference clock offsets, antenna pointing, and the acquisition of very long PN codes.
To facilitate LPI/LPD of item (c), the waveform must support very fast acquisition; the receiving node must be capable of reliably extracting data from the transmitting node within a small fraction of a second from when the transmission begins, so that bursts are short and covert. The initial bursts should be at the lowest possible power level and spread over a band that is wider than the signal bandwidth required. This fast acquisition should occur regardless of whether a node is stationary or mobile at rates of up to several thousand km/hr in any direction, and when relative velocity between nodes is neither known nor estimable before discovery.
Item (d) is that any node is capable of discovering any other node. This implies, for example, that if an FDD approach is adopted, wherein nodes that transmit in frequency band A and receive in frequency band B, can only be heard by nodes that receive in band A, then the nodes must be capable of switching their FDD polarity. In other words, if a hailing node sends discovery bursts in band A and receives no response, then it must be capable of switching to band B to search for nodes listening in that band. Correspondingly, a TDD system imposes the trivial constraint that nodes be capable of switching which time slot in which they transmit
Item (e) is desirable for discovery to avoid a need for a complicated and cumbersome clock availability requirement across the system. This complicates discovery in that discovery bursts may be sent at any time rather than in designated slots (thus enhancing LPD, LPI, and discovery range for mobile nodes closing on one another), and eliminates the need for hailing nodes to be synchronized with precision.
What is needed in the art is a receiver that can quickly acquire a node discovery signal or a reply to one (e.g., determine the PN code, phase, offset, and frequency). One particularly flexible code is described in a paper by Yingwei Yao and H. Vincent Poor, entitled A Two-Layer Spreading Code Scheme for Dual Rate CDMA Systems, IEEE TRANSACTIONS ON COMMUNICATIONS, vol. 51, No. 6, p. 873-879; Jun. 6, 2003. That paper describes a dual-rate direct sequence CDMA system using a variable spreading length two-layer P/N code. The authors claim advantageous results with simulations using recursive least squares receivers. The concept of codes with more than two layers is also disclosed. While the inventors herein have independently arrived at multi-layer codes similar to those of Yao and Poor, they have also developed a receiver that enables the advantages noted above that is novel over the recursive least squares receiver described by the above authors.